Global climate change is one of the, if not the, greatest challenges we face this century. A stable climate is necessary for sustainable economic development. Almost all climate scientists believe that the world is heating up because of human activities that emit greenhouse gases, particularly carbon dioxide, into the atmosphere. Few serious commentators doubt that urgent action is needed to prevent catastrophic changes in the climate.

There are two schools of thought about the best course of action. One wants to change society, a social revolution no less, to make it less consumerist and less materialistic. This sort of society, the argument goes, will use less energy and, therefore, emit smaller amounts of greenhouse gases into the atmosphere. The production of virtually all goods requires energy and the less money people spend the less energy they consume and the less greenhouse gases are emitted into the atmosphere.

The other school is less ambitious, and perhaps, therefore, more realistic. It relies on technology to solve the problem by developing, in the long term, a new relatively carbon-free source of energy, such as nuclear fusion reactors and, in the shorter term, developing a way to capture carbon dioxide emitted by power stations, transporting it by pipeline to a place for indefinite geological disposal, perhaps in deep underground mines or in trenches deep in the oceans. Geo-engineering methods to reflect the sun’s energy back into space – by, for example, the use of mirrors or whitening clouds – are also discussed.

Each of these two solutions has its difficulties. A sustainable social revolution would take generations to bring about. A technological fix, an attractive solution, would also take time and much money to overcome very difficult technical problems.

Nuclear Power as a Relatively Low Carbon Source of Energy

The world faces an increasing population and demands for higher living standards. These will inevitably bring about increasing demands for energy. In the shorter term, low carbon sources of electricity are seen to be the best way of satisfying these demands. Nuclear power is a relatively low carbon source of electricity.

Many political leaders are, therefore, looking to nuclear power as the main way forward to reduce the emissions of greenhouse gases and limit global climate change. The nuclear industry expects, therefore, that there will be a large increase in the global use of nuclear power for electricity generation.

Many worry about the prospect of the use of more nuclear power. Some remember the 1986 accident at the Chernobyl nuclear plant. Another catastrophic nuclear accident cannot be ruled out. The more nuclear plants there are the greater is the probability of an accident.

Another major concern is the disposal of large amounts of highly radioactive waste. So far, no publicly acceptable solution to the waste disposal problem has been forthcoming.

But perhaps the most serious problem associated with a nuclear renaissance is the possible proliferation of nuclear weapons. A fact of nuclear life is that civil nuclear technology and military nuclear technology are identical. Any country operating nuclear reactors could, if it chose to do so, use its nuclear capabilities to fabricate nuclear weapons. Moreover, a large terrorist group may well have the resources to acquire and detonate nuclear explosives, if it could acquire the nuclear material to do so.

In the words of US President Barack Obama, nuclear terrorism is "the single biggest threat to U.S. security, both short-term, medium-term and long-term. This is something that could change the security landscape of this country and around the world for years to come." He went on to explain that terrorist organizations like Al-Qaeda want to acquire a nuclear device, "a weapon of mass destruction that they have no compunction at using." (1).

The Increasing Global Use of Plutonium

Central to the terrorist risk is the fact that in a nuclear reconnaissance an increasing amount of plutonium will be used. Plutonium is a dual purpose material. It can be used both to fuel nuclear-power reactors and as the fissile material in nuclear weapons. A nuclear weapon containing a sphere of plutonium no larger than an orange could explode with the power equivalent to that of the explosion of about 20,000 tonnes of TNT – an explosion of this size destroyed the city of Nagasaki.

According to the International Atomic Energy Agency (IAEA), within 40 years, about 40 countries are likely to have access to fissile materials from their civil nuclear power programmes that could be used for nuclear weapons and competent nuclear physicists and engineers who could design and fabricate them. It has to be expected that some of these countries will take the political decision to become nuclear-weapon powers.

Because of the coming shortage of high-quality uranium, the nuclear industry will depend more and more on the use of plutonium to fuel nuclear reactors (2). After about 2030, according to current plans, a new generation of plutonium-fuelled reactors will be operated. These reactors, such as fast breeder reactors (FBRs), will, it is hoped, be the core of the nuclear renaissance.

Reserved Area

The world of the nuclear renaissance will be one containing a huge amount of separated plutonium (2), some of which will almost certainly fall into the wrong hands, including those of nuclear terrorists. The potential spread of nuclear weapons to terrorists clearly has very major implications for global security. Surprisingly, in spite of President Obama’s warning, it is receiving very little attention.

Nuclear Terrorism

There are number of nuclear terrorist activities that a terrorist group may become involved in, including: stealing or otherwise acquiring fissile material and fabricating and detonating a primitive nuclear explosive; making and detonating a radiological weapon, commonly called a dirty bomb, to spread radioactive material; and attacking a nuclear-power reactor or attacking a plutonium store at a reprocessing plant to spread radioactivity far and wide.

Of them, nuclear terrorists would probably prefer to set off a nuclear explosive, because of the great damage it would do, perhaps using a stolen nuclear weapon or more likely using a nuclear explosive fabricated by them from acquired plutonium.

Terrorists would be satisfied with a nuclear explosive device that is far less sophisticated than the types of nuclear weapons demanded by the military. Whereas the military demand nuclear weapons with predictable explosive yields and very high reliability, most terrorists would be satisfied with a relatively primitive nuclear explosive.

Dirty Bombs

The simplest and most primitive terrorist nuclear device is a radiological weapon or radiological dispersal device, commonly called a dirty bomb. A dirty bomb would consist of a conventional high explosive (for example, semtex, dynamite or TNT), some incendiary material (like thermite) surrounding the conventional explosive, and a quantity of a radioactive material, probably placed at the centre of the explosive.

When the conventional high explosive is detonated the radioactive material would be vaporised. The fire ignited by the incendiary material would carry the radioactivity up into the atmosphere. It would then be blown downwind, spreading radioactivity.

The use of plutonium in a dirty bomb would cause the greatest threat to human health, because of its very high inhalation toxicity, and the most extensive contamination.

The detonation of a dirty bomb is likely to result in some deaths but would not result in the hundreds of thousands of fatalities that could be caused by the explosion in a city of a crude nuclear weapon. Generally, the explosion of the conventional explosive would be the most likely cause of any immediate deaths or serious injuries. The radioactive material in the bomb would be dispersed into the air but would be soon diluted to relatively low concentrations.

If the bomb is exploded in a city, as it almost certainly would be, some people are likely to be exposed to a dose of radiation. But the dose is in most cases likely to be relatively small. A low-level exposure to radiation would slightly increase the long-term risk of cancer.

The main potential impact of a dirty bomb is psychological – it would cause considerable fear, panic and social disruption, exactly the effects terrorists wish to achieve. The public fear of radiation is very great indeed, some say irrationally so.

The explosion of a dirty bomb could result in the contamination of an area of a city and the surrounding areas with radioactivity. Areas as large as tens of square kilometres could be contaminated with radioactivity to levels above those recommended by national radiological protection authorities for the exposure of civilians to radioactivity. The area would have to be evacuated and decontaminated.

Decontamination is likely to be very costly (costing millions of pounds) and take weeks or, most likely, many months to complete. Radioactive contamination is by far the most threatening aspect of a dirty bomb.

Terrorist Nuclear Weapons

A nuclear weapon is a device that obtains its explosive yield from nuclear fission. A dirty bomb is not a nuclear weapon – there is no nuclear fission and no nuclear explosion.

Terrorists could make a nuclear weapon from either highly enriched uranium (HEU) or plutonium. The simplest nuclear explosive uses the 'gun technique’, in which a mass of enriched uranium less than the critical mass is fired, down a gun barrel, for example, into another less-than-critical mass of uranium. The sum of the two masses is greater than critical. When they join together a nuclear explosion occurs. (A critical mass is the minimum amount of a fissile material – HEU or plutonium – that will produce a nuclear explosion.)

Because almost all HEU is in military hands it would presumably be very difficult for terrorists to acquire HEU illegally. Therefore, terrorists may prefer to use plutonium.

Reserved Area

The gun technique cannot be used to assemble a super-critical mass of plutonium in a nuclear explosive device; implosion must be used. The implosion technique can, however, be used to assemble a super-critical mass of highly enriched uranium.

In a primitive nuclear explosive using the implosion design, a sphere of plutonium or highly enriched uranium, having a mass probably just less than critical so that it cannot sustain a fission chain reaction, is likely to be surrounded by conventional high explosives.

When exploded, the high explosive uniformly compresses the sphere of fissile material. The compression reduces the volume of the sphere of fissile material in the core and increases its density. The critical mass is inversely proportional to the square of the density. The original less-than-critical mass of fissile material will, after compression, become super-critical, and a fission chain reaction and nuclear explosion will take place.

If the fissile core were surrounded with, for example, a beryllium shell to reflect back fission neutrons that escape from the core, the critical mass will be significantly reduced.

Could Terrorists Fabricate a Nuclear Explosive?

If it could acquire the fissile material, a small group of people with appropriate skills could, in theory, design and fabricate a crude nuclear explosive (3). The size of the nuclear explosion from such a crude nuclear device is impossible to predict. But even if it were only equivalent to the explosion of a few tens of tonnes of TNT it would completely devastate the centre of a large city. Such a device would, however, have a chance of exploding with an explosive power of at least a hundred tonnes of TNT. Even one thousand tonnes or more equivalent is possible, but not very likely.

The Use of Reactor-Grade Plutonium to Fabricate Nuclear Weapons

There has been much discussion about whether or not a terrorist group (or a country) could use the plutonium recovered from today’s spent nuclear-power reactor fuel elements to fabricate a nuclear explosive device having a significant explosive yield.

Nuclear-weapon designers prefer relatively pure plutonium-239 for nuclear weapons. Plutonium containing 93 or more per cent of plutonium-239 and about 6 per cent plutonium-240 is called ‘weapons-grade’ plutonium. The plutonium produced in a commercial nuclear-power reactor operated for the most economical generation of electricity, called ‘reactor-grade’, typically contains about 60 per cent plutonium-239, about 20 per cent plutonium-240, about 15 per cent plutonium-241, and 5 per cent plutonium-241.

As J. Carson Mark, a former nuclear-weapon designer, explains, there are two major problems with using reactor-grade plutonium in a nuclear explosive device (4). Plutonium-240 has a high rate of spontaneous fission so that the device will continually produce many neutrons. One of these background neutrons may set off the fission chain reaction prematurely, called pre-initiation, causing the device to have a low explosive yield – perhaps the equivalent to the explosion of about 1,000 tonnes of TNT (a kiloton).

The second problem is the heat produced by the alpha-particle decay of plutonium-238. The amount of plutonium-238 in reactor-grade plutonium is about one or two per cent. This contributes 10.5 watts of heat per kilogram of reactor-grade plutonium, compared with 2.3 watts per kilogram of weapons-grade plutonium.

The design of a primitive nuclear explosive using reactor-grade plutonium would have to incorporate a method of dispersing the heat – such as the use of aluminium heat shunts. Otherwise, the plutonium would get very hot and become distorted or even melt.

More reactor-grade plutonium than weapon-grade plutonium would be required for a nuclear weapon. The bare sphere critical mass of reactor-grade plutonium is about 13 kilograms; that of weapons-grade plutonium is 10 kilograms. The use of reactor-grade plutonium to fabricate nuclear weapons is also described by Richard Garwin, a leading American nuclear-weapon expert (5 and 6).

In 1953, the British exploded a nuclear weapon, at its nuclear test site in South Australia, made from plutonium of a quality considerably below that of weapons-grade (7). In 1962, the United States conducted a similar nuclear-weapon test (8). These tests show that reactor-grade plutonium could very probably be used to fabricate a nuclear explosive device having a significant explosive yield.

Conclusion

US President Obama has recently announced a new initiative to reverse reliance on nuclear weapons in national security policies (9). His aim is to prevent the proliferation of nuclear weapons, reduce the risk of nuclear terrorism, and ultimately to achieve a nuclear-weapon-free world. The Obama approach is probably the best (perhaps the only politically realistic) way to reduce the risk of nuclear terrorism.

5. Richard L. Garwin, Reactor-Grade Plutonium Can be Used to Make Powerful and Reliable Nuclear Weapons: Separated plutonium in the fuel cycle must be protected as if it were nuclear weapons, Federation of American Scientists, August 26, 1998. www.fas.org/rlg/980826-pu.htm

6. Committee on International Security and Arms Control (CISAC) of the National Academy of Sciences, The Management and Disposition of Excess Weapons Plutonium, National Academy Press, Washington, DC (1994), pp32-33, text is available at www.nap.edu/readingroom/enter2.cgi?0309050421.html See also: American Nuclear Society, Protection and Management of Plutonium, Special Panel Report, August 1995, p. 25.

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